The invention relates to means for improving the geometry of the active picture area in a photoelectric tube, and in particular to means for circumventing the phenomenon known as "edge hooking" in television camera pickup tubes.
In photoelectric tubes, an electron beam scans a two-dimensional active picture area located within a surrounding non-scanned portion of the entire tube target. However, the entire target receives incident light, which accordingly, charges the entire backface of the target. Since the beam only reads out the active picture area, very large charge patterns remain in the surrounding non-scanned portion of the target. These charges bleed in the immediately adjacent active picture area, causing both horizontal and vertical "edge hooking," i.e., distortion along the top, bottom, left and right borders of the picture.
Some compensation for edge hooking is provided in the prior art by scanning the electron beam a small distance into the immediate boundaries surrounding the active picture area. This small distance, hereinafter termed "overscan," is determined by the velocity of the beam and the time available during the horizontal and vertical blanking intervals, immediately before and after the flyback period. The electron beam reads out, and thus discharges, a small boundary about the active picture area, corresponding to the overscan distance that the beam moves during the few microseconds available while traveling at normal scanning velocity. This in turn marginally decreases the amount of charge bleeding and thus edge hooking. However, edge hooking still persists and accordingly has been a constant problem in the design of television cameras.